saturate method
Implementation
TinyColor saturate([int amount = 10]) {
final p = RangeError.checkValueInInterval(amount, 0, 100, 'amount') / 100;
final hsl = toHSLColor();
final saturation = clamp01(hsl.saturation + p);
_color = hsl.withSaturation(saturation).toColor();
return this;
}